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| United States Patent |
5,270,350
|
|
Muller
, et al.
|
December 14, 1993
|
Crosslinked shaped dental articles
Abstract
The invention relates to a process for the production of crosslinked dental
mouldings, in particular for teeth made of plastic, the compositions used
therefore and the articles obtained.
| Inventors:
|
Muller; Michael (Bergisch-Gladbach, DE);
Podszun; Wolfgang (Cologne, DE);
Bebermeier; Gunther (Leverkusen, DE);
Richter; Roland (Leverkusen, DE)
|
| Assignee:
|
Bayer Aktiengesellschaft (Leverkusen, DE)
|
| Appl. No.:
|
824846 |
| Filed:
|
January 23, 1992 |
Foreign Application Priority Data
| Current U.S. Class: |
523/115; 523/113; 523/114 |
| Intern'l Class: |
A61K 006/08 |
| Field of Search: |
523/113,114,115
522/121,142
|
References Cited
U.S. Patent Documents
| 4230790 | Oct., 1980 | Hill | 522/121.
|
| 4337130 | Jun., 1982 | Abramjian | 522/142.
|
| 4379039 | Apr., 1983 | Fujimoto et al. | 522/142.
|
| 4379695 | Apr., 1983 | Orlowski et al. | 523/115.
|
| 4396476 | Aug., 1983 | Roemer et al. | 523/115.
|
| 4400159 | Aug., 1983 | Orlowski et al. | 523/115.
|
| 4406625 | Sep., 1983 | Orlowski et al. | 523/115.
|
| 4431421 | Feb., 1984 | Kawahara et al. | 523/115.
|
| 4491453 | Jan., 1985 | Koblitz | 523/115.
|
| 4639500 | Jan., 1987 | Kubo | 522/121.
|
| 4692396 | Sep., 1987 | Uchida | 522/121.
|
| 4904737 | Feb., 1990 | Sato et al. | 522/121.
|
| 4952241 | Aug., 1990 | Reiners et al. | 523/115.
|
| 4952614 | Aug., 1990 | Reiners et al. | 523/113.
|
| 4985343 | Jan., 1991 | Kushi et al. | 522/121.
|
| 5089291 | Feb., 1992 | Hayama et al. | 522/121.
|
Other References
Polymer Handbook-3rd, edition, VII/519-524, 544-557 Brandrup & Immergut
(1989).
|
Primary Examiner: Hoke; Veronica P.
Attorney, Agent or Firm: Sprung Horn Kramer & Woods
Claims
We claim:
1. A process for the preparation of a molded dental article, said process
comprising the following steps:
(a) shaping a curable solid composition into a dental mold using a
thermoplastic shaping process to yield a shaped curable solid composition;
removing the curable solid shaped composition from the mold; and
(b) crosslinking the shaped curable solid composition by
photopolymerisation to yield said molded dental article;
wherein said curable solid composition comprises the following ingredients:
(i) 40 to 90% by weight of a polymer component having a solubility
parameter of 8-12.5 (cal/cm.sup.3).sup.1/2 of 40-100% by weight of
non-crosslinked and 0-60% by weight of crosslinked polymer;
(ii) 10 to 60% by weight of monomer component containing 0 to 90% by weight
of monofunctional (meth)acrylic acid ester and 10 to 100% by weight of
polyfunctional (meth)acrylic acid ester; and
(iii) 0.1 to 5% by weight of photoactivator.
2. The method according to claim 1, wherein the curable composition
comprises:
(a) 40 to 90% by weight of polymer component having a solubility parameter
of 8-12.5 (cal/cm.sup.3).sup.1/2 of 40-100% by weight of non-crosslinked
polymer;
(b) 10 to 60% by weight of polyfunctional (meth)-acrylic acid ester;
and
(c) 0.1 to 5% by weight of photoactivator.
3. The method according to claim 1, wherein the thermoplastic shaping
process is injection molding.
Description
The invention relates to a process for the production of crosslinked dental
mouldings, in particular for teeth made of plastic, the compositions used
therefore and the articles obtained.
Teeth of non-crosslinked plastic can be produced from, for example,
polymethyl methacrylate by thermoplastic shaping. However, the teeth
obtained do not achieve the required use properties to their full extent,
so that their wear resistance and crazing resistance are inadequate.
Customary teeth made of plastic are produced by polymerisation of mixtures
of
______________________________________
35-50% by weight
of monomer liquid containing
80-95% by weight of methyl meth-
acrylate and 5-20% by weight of
ethylene glycol dimethacrylate as
the crosslinking agent
and
50-65% by wight
of powder of non-crosslinked
polymethyl methacrylate in the form
of bead polymers having an average
particle size of 30-120 .mu.m.
______________________________________
Dental materials for the production of teeth made of plastic which contain
(as well as non-crosslinked) crosslinked polymethyl methacrylate as the
powder are proposed in U.S. No. 4,396,377. The conventional mixture of
methyl methacrylate and a crosslinking agent such as ethylene glycol
dimethacrylate is used as the monomer liquid. Dental materials according
to U.S. No. 4,396,377 have the following composition:
______________________________________
0-50% of non-crosslinked polymer
10-70% of crosslinked polymer
20-66% of monomer (which does not act as a crosslinking
agent)
7-27% of crosslinking agent
______________________________________
EP-A 59,525 describes materials similar to those in the abovementioned
patent specification. The materials claimed have the following
composition:
______________________________________
0-50% of non-crosslinked polymer
10-70% of crosslinked polymer
2-30% of monomer (which does not act as a crosslinking
agent)
20-70% of crosslinking agent
______________________________________
Dental materials for the production of false teeth in which exclusively or
predominantly crosslinking agents are used as the monomer liquid and
exclusively cross-linked polymer having particular swelling properties is
used as the polymer component have been disclosed in DE-A 3,820,497 and
EP-A 110,092. These dental materials have the following composition:
______________________________________
5-35% of crosslinked polymer
0-40% of monomer (which does not act as a crosslinking
agent)
40-90% of crosslinking agent.
______________________________________
DE-A 2,403,211 describes dental compositions containing filler which are
characterised in that they contain exclusively microfine silicon dioxide
as the filler and bisGMA or specific urethane methacrylates obtained by
reaction of diisocyanates with hydroxylalkyl methacrylates as the monomer.
Teeth made of crosslinked plastic are produced by the socalled chemoplastic
process. In this process, mixtures of pulverulent polymer, monomer and
crosslinking agent are polymerised in metal moulds under pressure at
elevated temperature. The tooth can be built up in several layers
according to the natural model. A three-layered build-up with an enamel,
dentine and neck layer in which the individual layers differ by the
pigmentation and the crosslinking agent content is often chosen. Although
teeth which are obtained by the chemoplastic process are significantly
superior to those produced by thermoplastic processes in respect of wear
resistance, they still do not achieve the level required for posterior
teeth (see Ullmann's encyclopedia of industrial chemistry, Volume A8 Page
280, Weinheim, N.Y. 1987).
The mixture of polymer powder, monomer and crosslinking agent used in the
chemoplastic process is inhomogeneous, since the polymer particles, which
are usually spherical and have an average particle size of 40 to 100
.mu.m, are not completely dissolved. The cured dental moulding is
consequently also not completely homogeneous, but contains regions of
non-crosslinked polymethyl methacrylate. These non-crosslinked regions are
partly responsible for the inadequate wear resistance and solvent
resistance.
The chemoplastic process moreover has other disadvantages: the times needed
for filling, heating, polymerising, cooling and releasing from the mould
are long; under industrial production conditions, the cycle times are
about 20 to 50 minutes. Because of these long cycle times, a large number
of individual moulds, associated with high investment costs, is needed. In
addition, the energy used for heating and cooling the moulds is high.
Since the teeth have flashes after removal from the mould, expensive
subsequent working steps, such as, for example, tumbling and polishing,
are necessary. Overall, the chemoplastic process is far less economical
than the thermoplastic process.
The known chemoplastic process is severely limited in respect of the
possibility of varying the starting materials employed. Thus, only simple
alkylene dimethacrylates which partly dissolve the polymer powder at a low
temperature can be used as the crosslinking agent.
The object of the invention was to provide a process for the production of
crosslinked dental mouldings which does not have the disadvantages
described above. This object is achieved by a specific process which is
based on thermoplastic shaping and subsequent crosslinking by
photopolymerisation.
The invention thus relates to crosslinked dental mouldings and to a process
for the production of these dental mouldings, which is characterised in
that a mixture containing
______________________________________
a) 40 to 90% by wt.
of polymer component having a
solubility parameter of 8 to
12.5 (cal/cm.sup.3).sup.1/2 of 40 to 100% by
weight of non-crosslinked and 0 to
60% by weight of crosslinked
polymer
b) 10 to 60% by wt.
of monomer component containing 0
to 90% by weight of monofunctional
(meth)acrylic acid ester and 10 to
100% by weight of polyfunctional
(meth)acrylic acid ester
and
c) 0.1 to 5% by wt.
of photoactivator
______________________________________
is processed to shaped articles by a thermoplastic process and then
crosslinked by photopolymerisation.
The crosslinked dental mouldings according to the invention are, for
example, false teeth, crowns, bridges, veneer shells, inlays and onlays.
The new process can be used particularly advantageously for the production
of false teeth.
The polymer component (a) employed according to the invention has a
solubility parameter of 8 to 12.5, preferably 8.5 to 12
(cal/cm.sup.3).sup.1/2. The solubility parameters of important known
polymers and calculation methods for new polymer compositions are
described in the literature, for example in the Polymer Handbook 3rd
Edition, Brandrup und Immergut, John Weley and Sons, New York, 1989.
Examples of suitable polymers are polyesters, polycarbonates,
polyester-polycarbonate copolymers and vinyl polymers, such as
styrene-acrylate copolymers, styrene-acrylonitrile-acrylate terpolymers
and polyvinyl esters. Preferred polymers are homo- and copolymers of
(meth)-acrylic acid esters. Polymethyl methacrylate and copolymers of
methyl methacrylate with C.sub.1 -C.sub.8 -alkyl (meth)acrylates as the
comonomer are particularly preferred. Particularly suitable comonomers are
ethyl acrylate, n-butyl acrylate, iso-butyl acrylate, n-butyl
methacrylate, isobutyl methacrylate, ethylhexyl acrylate, n-octyl acrylate
and ethylhexyl methacrylate.
The monomer component (b) contains 0 to 90 % by weight, preferably 0 to 80
% by weight, particularly preferably 0 to 60 % by weight, of
monofunctional (meth)acrylic acid esters and 10 to 100 % by weight,
preferably 20 to 100 % by weight, particularly preferably 40 to 100 % by
weight, of polyfunctional (meth)acrylic acid esters.
Suitable monofunctional methacrylic acid esters are, for example, methyl
methacrylate, ethyl acrylate, n-propyl methacrylate, iso-propyl
methacrylate, n-butyl acrylate, n-butyl methacrylate, iso-butyl
methacrylate, n-hexyl methacrylate, ethylhexyl acrylate, ethylhexyl
methacrylate, n-octyl methacrylate, cyclohexyl acrylate, cyclohexyl
methacrylate, phenylethyl acrylate, phenylethyl methacrylate,
2-methoxyethyl methacrylate, triethylene glycol monomethacrylate,
3-methoxybutylmethacrylate, butoxyethyl acrylate, furfuryl methacrylate
and tetrahydrofurfuryl acrylate.
The term polyfunctional (meth)acrylic acid esters means esters of acrylic
acid and methacrylic acid having two or more polymerisable double bonds.
Esters of di- to octavalent alcohols having 2 to 30 carbon atoms may be
mentioned as preferred. Epoxide (meth)acrylates and urethane
(meth)acrylates are particularly preferred.
Examples which may be mentioned are (meth)acrylic acid esters of the
formula
##STR1##
in which
A denotes a straight-chain, branched, cyclic, aliphatic, aromatic or mixed
aliphatic-aromatic radical having 2 to 30 C atoms, which can be
interrupted --O--or NH bridges and can be substituted by hydroxyl, oxy,
carboxyl, amino or halogen,
R' denotes H or methyl and
n represents an integer from 2 to 8, preferably 2 to 4.
The compounds of the following formulae may be mentioned as preferred:
##STR2##
wherein R represents
##STR3##
Derivatives of tricyclodecane (EP-A 0,023,686) and reaction products of
polyols, diisocyanates and hydroxyalkyl methacrylates (DE-A 3,703,120,
DE-A 3,703,080 and DE-A 3,703,130) may also be mentioned. The following
monomers may be mentioned as examples:
##STR4##
A particularly preferred (meth)acrylic acid ester is so-called bis-GMA of
the formula
##STR5##
It is of course possible to employ mixtures of different monofunctional
and/or polyfunctional (meth)-acrylic acid esters.
the mixtures according to the invention in general contain 0.1 to 5%,
preferably 0.25 to 1%, of a photoactivator. The photoactivator consists of
a photopolymerisation initiator and if appropriate additionally a
coactivator.
Photopolymerisation initiators in the context of the present invention are
agents which form free radicals which trigger off free radical
polymerisation under the action of light, for example UV light, visible
light or laser light.
Photopolymerisation initiators are known per se (see, for example, Houben
Weyl, Methoden der organischen Chemie (Methods of organic chemistry),
Volume E 20, Page 80 et seq., Georg Thieme Verlag Stuttgart, 1987). They
are preferably mono- or dicarbonyl compounds, such as benzoin and
derivatives thereof, in particular benzoin methyl ether, benzil and benzil
derivatives, for example 4,4-oxydibenzil, and other dicarbonyl compounds,
such as diacetyl, 2,3-pentanedione and .alpha.-diketo derivatives of
norbornane and substituted norbornanes, such as, for example,
camphorquinone, metal carbonyls, such as manganese pentacarbonyl, or
quinones, such as 9,10-phenanthrenequinone and naphthoquinone.
In general, in addition to the actual photopolymerisation initiator, the
mixture also contains a coactivator which accelerates the
photopolymerisation. Known coactivators are, for example, amines, such as
p-toluidine, N,N-dimethyl-p-toluidine, trialkylamines, such as
trihexylamine, polyamines, such as N,N,N',N'-tetraalkylalkylenediamines,
barbituric acid and dialkylbarbituric acids.
Dimethylaminobenzenesulphonamides according to DOS (German
Offenlegungsschrift) 3,135,113 are also particularly suitable.
The ratio of photopolymerisation initiator to coactivator is in general 1:1
to 1:3, and in many cases a ratio of 1:2 can be used.
In addition to components a), b) and c), the mixture can of course contain
as further components all the additives which are usually used for the
production of dental mouldings.
It is thus possible to add UV stabilisers to avoid subsequent darkening
during ageing. A particularly suitable stabiliser is
2-hydroxy-4-methoxybenzophenone.
2-(2'-Hydroxy-5-methylphenyl)-benzotriazole may be mentioned as a further
example.
Pigments and dyestuffs which are known per se can be used to establish a
colour which is as natural-looking as possible.
The mixture of components a), b) and c) can be prepared by compounding in
high-performance mixing units, such as kneaders or twin-screw extruders.
The mixing temperature here is in general in the range from 110.degree. to
180.degree. C., preferably 120.degree. to 160.degree. C. It has been found
that no undesirable premature crosslinking occurs under these conditions.
Mixing can also be facilitated by addition of solvents, such as acetone,
methyl ethyl ketone, tetrahydrofuran, dioxane, ethyl acetate, methylene
chloride and chloroform. A preferred solvent is methyl ethyl ketone. After
the mixing operation, the solvent can be removed, for example, with the
aid of a devolatilisation extruder. The extrusion is advantageously
followed by a granulating step.
Shaping is carried out by thermoplastic processing, preferably by injection
moulding. The optimum temperatures of the injection moulding process
depend on the nature and composition of components a) and b) and can be
determined by simple preliminary experiments. For example, the following
temperatures have been determined as being particularly favourable for a
mixture of polymethyl methacrylate and bis-GMA:
______________________________________
Melt temperature 130-140.degree. C.
Die temperature 160-170.degree. C.
Mould temperature
30-40.degree. C.
______________________________________
In many cases, the optimum melt temperature is in the range from
110.degree. to 180.degree. C.
Commercially available injection moulding machines with, for example, tooth
moulds as the mould are suitable for the processing. Several layers can be
injected in succession by using blenders and by an appropriate design of
the mould. The bonding of the layers to one another is good. The cycle
time for an injection operation is about 10 to 60 s.
After release from the mould, the dental mouldings obtained have adequate
strength and dimensional stability, so that they are not deformed or
damaged during further working
The mouldings are crosslinked under the action of light. Light of various
wavelengths, for example visible light or UV light, is suitable in
principle. The emission spectrum of the light source and the spectral
sensitivity of the photoinitiator used must of course be matched to one
another.
The radiation sources used are powerful lamps or, particularly
advantageously, so-called light ovens which allow a good control of the
temperature and the use of an inert gas.
Photo-DSC (Differential Scanning Calorimetry) is particularly suitable for
characterising the photopolymerisation. Using this method, the heats of
polymerisation can be determined as a measure of the polymerisation
conversions for a given recipe as a function of the experimental
conditions. The polymerisation conversion in general increases as the
radiation intensity, radiation time and temperature increase.
It is particularly advantageous to increase the temperature during the
polymerisation, for example from 50.degree. C. to 90.degree. C. Both a
good dimensional accuracy and a high polymerisation conversion are
achieved in this manner.
The irradiation time needed depends on the intensity of the radiation
source and the layer thickness of the dental moulding. Customary
irradiation times are in the region of a few minutes, for example 1 to 15
minutes.
The irradiation can also be followed by after-treatment by heating
(annealing). This after-treatment by heating is advantageously carried out
at a temperature above the glass transition temperature of polymer
component (a). A temperature of 120.degree. to 150.degree. C. and a heat
treatment time of 3 to 12 hours is particularly suitable for shaped
articles containing polymethyl methacrylate. Any stresses present are
removed and the residual monomer content, which is low according to the
invention, is reduced still further by this heat treatment.
The shaped articles produced by the process according to the invention have
a particularly high mechanical strength and resistance to solvents. The
significantly improved wear resistance compared with materials of the
prior art is to be emphasised in particular.
Example 1
Preparation of granules
The components listed below are mixed homogeneously and processed to
granules having a diameter of about 5 mm in a compounding screw extruder
with a downstream granulator:
______________________________________
2,585.60 g of polymethyl methacrylate bead polymer
(molecular weight Mw 450,000)
976.00 g of bis-GMA
2,2-bis-[p-(2'-hydroxy-3'-methacryloxy-
propyl)-phenyl]-propane
416.00 g of triethylene glycol dimethacrylate
5.60 g of camphorquinone
14.00 g of p-dimethylamino-benzenesulphonic acid
N,N-diallylamide
2.80 g of 2,6-di-tert.-butylkresol.
______________________________________
The melt temperature in the extruder was 140.degree. C. and the viscosity
of the melt at this temperature was 2,000 Pa.s, measured at a shear rate
of 100/s. The granules obtained were protected from the action of light
during preparation, storage and further processing.
Example 2
Injection moulding
The granules from Example 1 were injection-moulded to teeth in metal moulds
using an injection moulding machine of the Engel 433 type. The following
experimental conditions were observed during this process:
______________________________________
Melt temperature: 140 to 150.degree. C.
Die temperature: 160.degree. C.
Mould temperature: 35 to 40.degree. C.
Locking force: 600 kN
Screw speed: 200/minute
Injection time: 4 seconds
After-pressure time:
12 seconds
Cooling time: 20 seconds
Pause time: 3 seconds
Cycle time: about 40 seconds
______________________________________
Example 3
Investigation of the photopolymerisation
The photopolymerisation of the mixture from Example 1 was investigated in a
photo-DSC, by a procedure in which cylindrical test specimens (diameter: 5
mm, height: 1 mm) produced at 140.degree. C. were irradiated with a 75 W
halogen lamp with a heat protection filter in the DSC apparatus. Both the
reaction enthalpy (delta H, as a measure of the polymerisation conversion)
and the time taken to reach the maximum rate of reaction (t-max) were
measured at 50, 70.degree. and 90.degree. C.:
______________________________________
Measurement temperature
Delta H t-Max.
______________________________________
50.degree. C. 21 J/g 5.6 min.
70.degree. C. 36 J/g 2.9 min.
90.degree. C. 51 J/g 2.0 min.
______________________________________
The reaction enthalpy of 51 J/g corresponds to virtually complete
conversion, which was confirmed by infrared spectroscopic determination of
the double-bond content.
Example 4
Photopolymerisation
Injection-moulded teeth from Example 2 were irradiated at 50.degree. C. for
8 minutes and at 90.degree. C. for a further 5 minutes and then
heat-treated at 130.degree. C. for 3 hours in a light oven fitted with
halogen lamps (output 75 W) while flushing with nitrogen. The resulting
teeth made of plastic had good hardness, high wear resistance and good
resistance to solvents.
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